The present invention relates to implantable medication infusion devices, and, more particularly to such devices which are arranged to provide a continuous unprogrammed flow of medication into the body.
Medication infusion devices of the continuous flow type shown in Blackshear et al U.S. Pat. No. 3,731,681 and in the article by B. M. Wright entitled "A Portable Slow Infusion Capsule" in the Journal of Physiology March 1965, Vol. 177 No. 1 (Cambridge University Press) and devices similar to the Blackshear patent are presently available on the market. In such devices a relatively constant pressure is exerted on a flexible diaphragm or bellows which contains a reservoir of medication. The pressure exerted on the bellows is above body pressure so that medication is forced out of a long capillary tube, which is used as a flow limiting resistance device, and delivered to the infusion site within the body. This capillary tube is usually made of stainless steel and a length of 50 feet may be required to provide sufficient resistance to flow for desired delivery rates of medication even when the tube is fabricated to the minimum practical inside diameter of 0.004 inches. This stainless steel capillary tube is wrapped around the outside of the implantable device, or in a recess in the outside of the housing of such device as shown in Blackshear U.S. Pat. No. 3,731,681, and the end of the capillary tube is connected to a flexible catheter which is positioned at the infusion site in the body.
When such a long stainless steel capillary tube is used the medication remains in contact with the stainless steel for many hours, or even days, at relatively low infusion rates and this long residence or dwell time within the capillary tube causes problems of compatibility with the medication, particularly when a medication such as insulin is used. When a stainless steel capillary tube is used the medication is also more likely to precipitate out and clog the capillary tube. While titanium is more compatible to the medication and is less likely to clog, it is impossible to fabricate a titanium capillary tube of such small diameter. When it is desired to provide different flow rates with such a stainless steel capillary tube, each device must be tested separately after it is manufactured and the length of the capillary tube is trimmed down and then retested to get a particular infusion rate.
It is often necessary to mix the medication with a high viscosity diluent for use in existing continuous infusion devices in order to limit the capillary tube to a practical length, such as 50 feet. Using a high viscosity diluent makes the medication less potent while increasing the viscosity of the medication solution, and since capillary tube length is directly proportional to medication solution potency and inversely proportional to its viscosity, both of these effects reduce the length of capillary tube. For example, it is not feasible to deliver standard, undiluted 100 unit insulin to diabetic patients with existing devices. By diluting 100 unit insulin with 80% glycerol it is possible to use these devices but they must have large (typically 30 to 50 milliliters) reservoirs to contain the diluted medication and such a large reservoir requires the use of a flexible multiple convolution bellows rather than a single diaphgram, thus making the devices large and heavy and not suitable for implantation in children and less than average size adults. Such bellows type devices suffer from the additional disadvantage that they have many crevices ano a much higher residual and unusable medication volume than the diaphragm type of reservoir.
The infusion rate of existing continuous infusion devices also varies considerably due to changes to patient body pressure and changes in patient body temperature. Body pressure change, which varies the pressure at the outlet of the catheter and capillary tube restrictor, is due to variations in altitude which changes the ambient atmospheric and body pressure of the patient. Also, normal small changes (as well as abnormal changes due to sickness) in patient body temperature change the vapor pressure of the medication reservoir pressurant, thus changing the pressure at the inlet of the capillary tube restrictor. Therefore, both altitude and temperature changes act to vary the pressure drop across the capillary tube restrictor and the infusion rate through the capillary tube, which is directly proportional to this pressure drop. For example, if the patient travels from sea level to 10,000 feet above sea level, the drug infusion rate of existing devices increases by a factor of 2.5. If the patient temperature should also happen to increase from 97.degree. F. to 101.degree. F., the combined effect of the altitude and temperature changes would increase the infusion rate by a factor of 3.5. This large infusion rate variation can severely limit the clinical effectiveness and benefits for many continuous drug infusion treatments.
In Barth application Ser. No. 616,658 filed June 4, 1984 an integral fluid filter and capillary arrangement is disclosed in which the capillary is formed by a groove etched in the surface of silicon substrate by conventional semiconductor processing techniques and a glass plate is bonded to said surface of the substrate to form a long capillary groove of very small cross sectional area. A plurality of parallel grooves of smaller cross sectional area are also etched in the substrate surface to provide a comb filter at each end of the capillary groove.